Various polymerization reactions are known using monomers in the form of droplets dispersed in a continuous phase (generally an aqueous phase) with stabilizers of reactive stabilizer type.
In general, the reactive stabilizer used in these reactions is a molecule which can, firstly, stabilize the droplets of the dispersed phase containing the monomers, and which is also able to be engaged in the polymerization reaction. Such reactive stabilizers have, inter alia, the advantage of totally or partly replacing the additional surfactants that it is otherwise necessary to use in order to stabilize the dispersed phase, and they accordingly lead to polymer dispersions (latices) that are advantageous in the sense that they are free of such surfactants or at the very least in that they have a reduced content of such surfactants.
Such reactive stabilizers have especially been proposed for performing emulsion polymerizations of hydrophobic monomers of vinyl type, which are molecules of hydrophilic or amphiphilic nature typically comprising a hydrophilic polymer chain and a reactive group that is capable of inducing a controlled radical polymerization reaction of the monomers, for example of ATRP (atom transfer radical polymerization) or NMP (nitroxide-mediated polymerization), or RAFT or MADIX type in the presence of free radicals, with, for example, a group of xanthate type (bearing —S(C═S)O— functions) at the end of the chain. These reactive stabilizers ensure, firstly, the stabilization of the emulsion of droplets of the dispersed phase due to their hydrophilic nature, and they act, secondly, as polymerization control agent, typically via reversible addition-fragmentation transfer processes, via which the polymer chains grow from this control agent by gradual consumption of the monomers contained in each droplets, according to a process that is well known per se, in which the polymer chains have a “living” nature, in the sense that each step of incorporation of a monomer unit into the chain leads to a polymer which still bears a reactivable chain end for the subsequent incorporation of another monomer unit according to the same addition-fragmentation process (for further details regarding controlled or “living” radical polymerization and the production of chains growing from the transfer agent, reference may be made especially to the Handbook of RAFT polymerisation, Ed Barner-Kowollik C. Wiley-VCH 2008 or alternatively to the processes described in WO 96/30421, WO 98/01478, WO 99/35178, WO 98/58974, WO 00/75207 and WO 01/42312, WO 99/35177, WO 99/31144, FR2 794 464 or WO 02/26836).
With the reactive stabilizers of the abovementioned type, which induce a controlled radical polymerization, the formation of living polymer chains in the dispersed droplets is obtained, with an incorporation of the stabilizer into these polymer chains, from the very start of the polymerization, but while nevertheless conserving stabilization of the emulsion throughout the polymerization without necessarily having to use an additional surfactant, the hydrophilic chain schematically remaining permanently at the periphery of the droplets as illustrated in the attached FIG. 1.
Reactive stabilizers of the abovementioned type have especially been proposed in the processes described in the following publications:    Nitroxide-Mediated Controlled/Living Free-Radical Surfactant-Free Emulsion Polymerization of Methyl Methacrylate using a Poly(Methacrylic Acid)-based Macroalkoxyamine Initiator.    C. Dire, S. Magnet, L. Couvreur, B. Charleux    Macromolecules 42(1), 95-103 (2009)    PEO-based Block Copolymers and Homopolymers as Reactive Surfactants for AGET ATRP of Butyl Acrylate in Miniemulsion.    W. Li, K. Min, K. Matyjaszewski, F. Stoffelbach, B. Charleux    Macromolecules 41, 6387-6392 (2008)    Effective ab Initio Emulsion Polymerization under RAFT Control    Christopher J. Ferguson, Robert J. Hughes, Binh T. T. Pham, Brian S. Hawkett, Robert G. Gilbert, Algirdas K. Serelis, and Christopher H. Such    Macromolecules (2002)    Surfactant-free controlled/living radical emulsion (co)polymerization of n-butyl acrylate and methyl methacrylate via RAFT using amphiphilic poly(ethylene oxide)-based trithiocarbonate chain transfer agents.    J. Rieger, G. Osterwinter, C. Bui, F. Stoffelbach, B. Charleux    Macromolecules (2009)Although advantageous in absolute terms, one of the limitations of the reactive stabilizers described hitherto is that they are suited only to the polymerization of certain vinyl monomers, essentially of methacrylate and alkyl acrylate type.
One aim of the present invention is to provide a dispersed-phase polymerization process suited to the use of a large number of hydrophobic monomers and especially suited to the dispersed-phase polymerization of vinyl esters and of hydrophobic N-vinyl monomers.